This invention relates to a process and a device for injection moulding articles comprising a number of layers of different plastic materials, in which the different plastic materials are sequentially and/or coaxially injected (from concentric chambers along a common axis of introduction) in plasticised form, into a mould.
In particular, the invention relates to injection moulding multilayered thin-walled objects with a wall thickness of 0.5-3.0 mm.
An example of such a process is described in British Pat. No. 1,475,898, wherein first, second and third materials are sequentially injected into a mould cavity to provide an article having an external layer, an intermediate layer and a core. After a first material has been injected into the mould cavity it will flow into the cavity until it fills the mould. Before the first material has solidified, a second material is injected into the mould which will penetrate the first material, forcing it back towards the walls of the mould in such a manner that the first material forms the external layer and the second material forms an intermediate layer. The third material is then injected into the mould and will penetrate the second material to form the core.
Since the materials are injected into the mould from a single discharge injection unit, the flow of materials from the three concentric chambers to the plasticising units and the channels to the discharge unit are controlled by valves.
However, when an injection moulding process is used for the production of thin-walled objects, the mould must be filled within a very short time interval, i.e. about one second. Since the walls of the mould are relatively cooler than the plasticising and discharge injection units, the thin, plastic layers of material will begin to solidify and if the material solidifies or hardens too quickly, the materials will not be distributed uniformly in the mould.
Further, since valves are used to control the flow of the materials, the introduction of those materials into the mould imposes a further limitation on the reduction of the injection time.
Other problems associated with using such a device is that the plasticising/injection units and bodies cannot be moved rapidly, in order to reduce the time intervals between injections, and this results in a non-uniform distribution of the materials. In addition, such devices are difficult to manufacture when three or more different materials are to be injection moulded. Moreover, this problem occurs if a multi-step velocity profile is desired.
The present invention provides a process and a device which overcomes the above-mentioned problems.
Therefore, an object of the invention is to inject the different materials into the mould in as short a time interval as possible.
The process according to the present invention is characterized in that the different plastic materials are brought into a cavity or hollow chamber/injection unit through a distributor in a preselected order and amounts prior to injection of the materials from the cavity or hollow chamber/injection unit into the mould.
By application of this process, a significant reduction in the amount of time it takes to fill the mould is achieved, since the mould is filled from a single injection unit.
Further, the period of time which is available for filling the cavity including the cooling time (per cycle) in the process takes place over a shorter period of time than in the processes described in the prior art. Therefore, a number of multilayered compositions can be produced.
The shorter period of time required to fill the mould is also advantageous from an economic standpoint.
An example of this process is described in British Pat. No. 1,420,948 and U.S. Pat. No. 4,029,841, wherein the skin and core materials are positioned in a heating zone, heated and forced into the mould to form articles made of different polymer compositions.
However, British Pat. No. 1,420,948 discloses the use of taps to control the flow of the skin and core materials into the core material barrel and to permit injection of those materials into the mould.
U.S. Pat. No. 4,029,841 discloses that any injection moulding apparatus can be employed which permits a thermoplastic material to be injected into a mould cavity subject to the proviso that the equipment employed force the thermoplastic melt into the mould cavity in response to a force transmitted to the polymer melt by a nonrotating plunger contained within the barrel of an injection moulding machine as opposed and in contradistinction to the force transmitted to a polymer melt as a result of the rotation and thrust of the screw in the barrel of an injection moulding apparatus. However, a conventional injection moulding apparatus would also involve the use of a valve or tap.
Therefore, each of these methods imposes a further limitation on the reduction of the injection time.
In the process according to the present invention, the plastic materials are injected through a distributor comprising a plurality of disc shaped sections into a cavity or hollow chamber/injection unit after the plasticising phase and subsequently injecting the plastic materials in the cavity or hollow chamber/injection unit through the distributor and into the mould and, thus, form uniformly arranged layers within the mould without affecting the composition of those layers.
An advantage in using such a process is that relatively simple and inexpensive equipment can be used to fill the mould, i.e., a number of plasticising units and one injection unit.
The plastic materials are brought into the cavity or hollow chamber/injection unit in a preselected order and amounts by using a distributor. The cavity or hollow chamber/injection unit serves as a cavity or hollow chamber to receive and briefly hold and store the plastic materials and also as a means to inject those materials into the mould.
The distributor comprises a number of elementary modules, each capable of creating a particular combination and configuration of plastic materials in the cavity or hollow chamber/injection unit.
By this manner an article comprising many different layers can be made by simply moving the modules.
The elementary modules comprise a number of disc shaped elements provided with radially extending borings or holes. The disc shaped elements are arranged in a stack which can be moved in an axial direction with the aid of moving devices.
Each disc shaped element can be moved both axially as well as rotating around an axis to align the openings (borings) with the plasticising units in order to fill each disc with a particular plastic material and to discharge the material through an outlet to fill the cavity or hollow chamber/injection unit. This procedure is continued by moving the discs in the axial direction to add other plastic materials from a plasticising unit through the distributor into the cavity or hollow chamber/injection unit. After the cavity or hollow chamber/injection unit is filled with a preselected combination of plastic materials, the contents are injected into the mould through the distributor using a disc that connects the cavity or hollow chamber/injection unit to the feed channel of the mould.
The plasticising units and/or injection units can be connected to the cavity or hollow chamber/injection unit through the borings. Further, the borings may also be located concentrically along part of their length to enable the coaxial introduction of a number of materials.
The present invention also relates to a device for injection molding articles comprising a number of layers of different plastic materials, wherein the device comprises a mould, a distributor, a number of plasticising units and at least one injection unit.
The device according to the present invention comprises a cavity or hollow chamber/injection unit, which is connected to the plasticising units through a distributor. The cavity or hollow chamber/injection unit is further provided with an injection unit for injecting the contents of the cavity or chamber through the distributor into the mould.
By using a cavity or hollow chamber provided with an injection unit, it is possible to fill the mould over a shorter period of time.
The device of the present invention further comprises a distributor containing a number of elementary modules, with which each plasticising unit can be connected, either separately or in combination with another unit to the cavity or hollow chamber/injection unit.
In this manner a device can be made which is suitable for manufacturing an article with the following properties, such as a particular thickness of each layer, the number of layers, the position of a particular layer in the article and so forth by simply positioning or moving the modules in the distributor.
The distributor comprises a number of disc-shaped elements, provided with radially extending borings, which elements are arranged to form an elongated body, which can be moved by means of moving devices and can be fixed in a discrete position. The outer circumference of each disc-shaped element may be shaped like a regular polygon (triangle, quadrangle, etc.) or preferably a circle.
The process and device according to the invention are further illustrated in the drawings, described below.